1. Field of the Invention
The present invention concerns a filter element for a pressure filter of the kind where a substantially horizontal wall in a fluid tight manner separates the interior of a pressure vessel in a lower inlet compartment and an upper outlet compartment, and where at least one filter element having a filter medium is suspended from the wall such that filtering takes place from the inlet compartment up through the filter element to the outlet compartment.
2. Description of Background Art
One type of such filter including a plurality of filter elements depending from the horizontal plate and having shapes of perforated tubes covered with filter cloth is commonly referred to as a CLARIFIL filter and is used in recausticizing plants in the pulp and paper industry for liquor clarifying. From time to time each filter element must be backwashed in order to remove solids deposited on the filter cloth. Backwash takes places by causing a backwash liquid to flow from the outlet compartment, down through the filter element and into the inlet compartment flushing solids off the filter cloth. Typical for filters of the CLARIFIL-type is that filtrate, i.e., clarified liquor, is used as a backwash liquid. This, of course, involves a loss of filtrate, since the backwash filtrate dilutes the solids deposited on the filter cloth and accompanies them to a solids outlet. It is desirable, thus, to perform backwash in a CLARIFIL-type filter with another liquid than filtrate, preferably water, to achieve a higher degree of separation.
Other types of filters use a liquid other than filtrate, normally water, to perform backwash. In such filters, the filter elements are emptied from filtrate before backwash is performed, typically by blowing air through the filter elements, thus displacing filtrate contained within the filter elements. It is a desire to convert a CLARIFIL-type filter such that backwash with water would be possible, but a CLARIFIL-type filter cannot be emptied from filtrate by displacing filtrate upwards in the way described.
A measure of the effectiveness of a filter element is the degree of perforation, i.e., the ratio between the sum of the areas of all perforations and the entire area of a filter tube. In a CLARIFIL-type filter the degree of perforation is typically not above 30%, i.e., the open area under a filter cloth available for filtration is reduced to 30%.
Another measure to is the ratio between filter area (A) and interior volume (V) of a filter element. For a typical filter tube of a CLARIFIL-type filter element having an outer diameter of 60 mm, the relation A/V is about 6.
Further, in a CLARIFIL-type filter there is a tendency that the filter cake obtains a conical shape, i.e., that more solids are deposited close to the upper end of a filter tube than close to the free end of the tube. This may result in that solids deposited on adjacent tubes merge into a hard solid block that is impossible to remove by ordinary backwash, but necessitates disassembling of the filter for mechanical removal of the block of solids.